This invention relates generally to drive arrangements for variable speed three phase inverter fed induction motors and, more particularly, to a drive arrangement for motors of the type described which is fault tolerant.
Variable speed three phase inverter fed induction motor drive arrangements are extremely competitive in terms of cost and performance. However, in many aerospace applications, these arrangements have only limited usefulness since they are not fault tolerant. That is to say, a failure in one of the phases usually means the loss of the drive arrangement's starting capability.
In order to achieve fault tolerance, redundant motor windings and/or redundant inverter circuits are often required. Alternatively, in order to achieve the desired fault tolerance, three phase inverter fed induction motors have been replaced by switched reluctance machines. While the inverter topology of switched reluctance machines is indeed fault tolerant, the inherently high inductance of these machines makes them unsuitable for high performance servo applications as is often required for aerospace purposes.
The present invention uses the space vector concept and associated symmetrical component arrangements to render variable speed three phase inverter fed induction motor drive arrangements fault tolerant. Indeed, by utilizing the space vector concept, a motor of the type described can be operated (even started) with one of its three phases open and/or disconnected due to a fault.
It will be appreciated that a necessary condition for the drive arrangement contemplated by the present invention is that the current for each motor phase be controlled individually. This requirement implies that the motor neutral must be accessible and "grounded." If the motor neutral is not grounded, then, in the case of an inoperative motor phase, i.e. an open circuit, the currents in the remaining two phases are not independent.
The neutral of an induction motor supplied from a three phase inverter cannot be "grounded" to the center point of a DC link because in a normal, balanced three phase operation, the oscillating neutral phenomenon well known to those skilled in the art introduces large circulating currents through the motor and the inverter. If, however, a three phase induction motor with an accessible and "grounded" neutral is supplied from three independent single phase inverters, then the resulting drive arrangement can be made fault tolerant.
In a balanced three phase operation, the stator currents and the associated rotating field can be described in terms of their space vectors. If the neutral of the machine is ungrounded, the space vector fully defines the instantaneous phase quantities. If the neutral of the machine is "grounded," then in addition to the space vector, the zero sequence quantities of the stator current or flux must also be defined.
If current in one of the phases becomes zero due to a fault, the current regulators in the remaining two phases must operate according to a pre-defined control law to maintain a balanced rotating field, as will be understood by those skilled in the art.
To appreciate the space vector features of the present invention, it will first be understood that "vector diagrams" have long been used to graphically describe the performance of electrical machines. The term "vector diagram" has generally been replaced by the term "phasor diagram." As long as the voltages and currents represented in these diagrams are steady state sinusoidal quantities, there is little distinction between the terms. The primary feature of a phasor diagram is that it can display the steady-state phase difference between a voltage and its associated current.
When electrical quantities are non-sinusoidal and/or in a transient state, the phase relationship between a voltage and its associated current can vary from instant to instant and "phase shift" has no meaning at a given instant. The circumstances become even more complex in the case of three phase electrical machines, since the voltages and currents of the individual phase windings, in both the machine stator and rotor, can vary independently of each other. Thus, simple phasor diagrams are not sufficient to describe the aforementioned non-sinusoidal and/or transient phenomena. For this reason space vectors (also referred to as three-phase vectors or Park vectors) are introduced.
Space vectors described as aforenoted and as applied to a three phase electrical machine are described in the text "Transient Phenomena In Electrical machines" by P. K. Kovacs, published by Elsevier in 1984, the same being incorporated herein by reference. It should be noted that under steady-state conditions a space vector degenerates into a phasor.
The applicants herein are aware of the following prior art relating generally to the present invention: U.S. Pat. No. 4,367,522 which issued to Forstbauer, et al on Jan. 4, 1983 (U.S. Class 363,137); U.S. Pat. No. 4,800,327 which issued to Fujioka, et al on Jan. 24, 1989 (U.S. Class 318/798); and U.S. Pat. No. 4,999,561 which issued to Kaga, et al on Mar. 21, 1991 (U.S. Class 318/812).
The Forstbauer '522 patent discloses a three phase inverter arrangement wherein three pulse controlled, free running, single phase inverters are connected at their inputs to a DC voltage source. Each of the converters is controlled by an associated pulse control device containing a vector oriented control device and a control unit.
The Fujioka '327 patent discloses a three phase induction motor control method including the step of sensing voltage at a DC link section. The output of the motor is controlled so as to be constant in a conventional vector control type control circuit.
The Kaga '561 patent discloses a variable speed driving system for a three phase induction motor incorporating three phase inverters, a control mode judging circuit and a voltage difference detection circuit. The Kaga system equalizes the output powers of the series connected three phase inverters.
None of the aforementioned patents teach the particular structural arrangement herein disclosed as will be discerned from the description which follows.
It is to be further noted that in a recent publication entitled "Disturbance Free Operation Of A Multi-Phase Current Regulated Motor Drive With An Open Phase" by J. Fu and T. A. Lipo, published in the IEEE Transactions on Industry Applications, Volume 30, No. 5, September/October, 1994, pages 1267-1274, a fault tolerant arrangement is achieved with a three phase inverter and a WYE connected induction motor. However, for a three phase motor the Fu and Lipo arrangements need a center tapped DC link with an associated capacitor bank to which the neutral of the induction motor can be connected, and in this respect the Fu and Lipo arrangement is different than the arrangement herein disclosed.